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  #31  
Old 12-25-2018, 02:42 PM
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RV7A Flyer RV7A Flyer is offline
 
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Just out of curiousity...what do people think is happening to cause the FF transducer to read "a little high" when the boost pump is on? All the thing does is measure, via a little impeller sticking into the flow, the amount of fuel *flowing past it*. It doesn't "know" that the boost pump is on.

I guess, in some weird way, depending on where in the system the thing is mounted, it might see some odd pulsing somehow. But, e.g., if it's mounted between the servo and the spider, I don't see any way that it could register "a little high" because of an increase in *pressure* going to the servo from the boost pump.

BTW, FWIW, 600 hours on my red cube and not a hint of trouble. Mounted between the servo and the spider, via a bracket to sump bolts, firesleeved.
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  #32  
Old 12-25-2018, 03:06 PM
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I'm not qualified to render an opinion on electronics quality, but the red cubes's circuit board is potted in such a way that vibration should not hurt it.



The only moving part is the rotor. It spins on a jewel in the main body, and another in the rotor cover.



Here's the rotor in the main body. The arrow points to what I assume to be either a light source or a receiver. The little tabs on the rotor interrupt a light beam between it and...



...its counterpart located in the cover.



The device cannot be dismantled without damaging it, which appear to be by design, so cleaning the rotor and its chamber is only possible by flushing. I can see how some loose fuel lube might gum up the rotor, but can't think of much else an owner might do beyond hit it with a voltage spike.
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Last edited by DanH : 12-25-2018 at 06:17 PM.
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  #33  
Old 12-25-2018, 03:27 PM
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SmilingJack SmilingJack is offline
 
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Fuel lube???

Seems like a good theory. A few months back I had an issue with my RED CUBE where it fell to Zero intermittently . Then after a few hours it came back.

Maybe it was a little fuel lube I placed on the o-ring of the fuel caps...???

I didn’t like the stickyness amd switched to vasiline. It works much better and “fingers crossed” I don’t have any future RED CUBE issues.
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  #34  
Old 12-25-2018, 04:12 PM
PilotjohnS PilotjohnS is offline
 
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Quote:
Originally Posted by RV7A Flyer View Post
Just out of curiousity...what do people think is happening to cause the FF transducer to read "a little high" when the boost pump is on? All the thing does is measure, via a little impeller sticking into the flow, the amount of fuel *flowing past it*. It doesn't "know" that the boost pump is on.

I guess, in some weird way, depending on where in the system the thing is mounted, it might see some odd pulsing somehow. But, e.g., if it's mounted between the servo and the spider, I don't see any way that it could register "a little high" because of an increase in *pressure* going to the servo from the boost pump.

BTW, FWIW, 600 hours on my red cube and not a hint of trouble. Mounted between the servo and the spider, via a bracket to sump bolts, firesleeved.
I suspect the pulse from the fuel pump causes the rotor to jump back and forth sending extra pulses to be counted. I also suspect the fuel servo will smooth these out making those installations more accurate with the fuel pump on. Jmho ymmv
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  #35  
Old 12-25-2018, 08:37 PM
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[quote=FasGlas;1311620 The only thing left is to scope the pulses to isolate the cube from the RDAC. Always something............[/QUOTE]

When mine was acting up I started thinking about a wiring issue between the Red Cube and the Dynon EMS. On the phone with tech support they mentioned a quick way to test the wiring is to ground the sensor wire running from the Dynon system. It seems the Red Cube is grounding the sensor wire and Dynon system was measuring how long it was grounding. So if I grounded the sensor wire and got a reading on fuel flow the Red Cube was the problem, if no reading the problem was the wiring. I do not know, but it seems your system must be measuring the Red Cube data in the same manner.

As your problem is intermittent it would seem to indicate a problem with that aspect of the wiring...or another bad fuel flow transducer. Hope that helps in some way.
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  #36  
Old 12-26-2018, 08:17 AM
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Quote:
Originally Posted by sahrens View Post
When mine was acting up I started thinking about a wiring issue between the Red Cube and the Dynon EMS. On the phone with tech support they mentioned a quick way to test the wiring is to ground the sensor wire running from the Dynon system. It seems the Red Cube is grounding the sensor wire and Dynon system was measuring how long it was grounding. So if I grounded the sensor wire and got a reading on fuel flow the Red Cube was the problem, if no reading the problem was the wiring. I do not know, but it seems your system must be measuring the Red Cube data in the same manner.

As your problem is intermittent it would seem to indicate a problem with that aspect of the wiring...or another bad fuel flow transducer. Hope that helps in some way.
The way the Red Cube works, and probably the Floscan, it's an open collector output (switch). The LED light source, in the cube, is on one side of the slotted wheel and the phototransistor is on the other side. As the wheel spins it shines light on the transistor in a "Pulse" fashion. When the transistor gets that light it opens the circuit. Connected to the signal wire, on the RDAC end, is "Bias" voltage. This bias voltage is what creates the pulses from 0 ~ 5 volts. The pulse is open bias voltage and shorted closed transistor voltage. In the case of the MGL RDAC the bias voltage comes from a 5.6K resistor that comes with the Red Cube and is mounted between the 5 volt connector and the Fuel Flow sensor connector (FF1 or FF2). EI, JPI and others apply the bias voltage internally but they all work the same.
Connecting a scope to the sensor signal wire should show a square wave like pulse as the fuel flows through the sensor. In my case 18000 pulses = 1 liter of fuel or 68,000 per gallon. The math is different per manufacture. This is called the "K" factor.
Any random pulses from stray interference like spark plug wires, etc, will also show on the scope. Missing pulses as well. Depending on the electronics the pulses could be canceled out by interference, depends on the filtering designed into the converter circuits. Other factors like multiplex timing, etc could also effect the RDAC output to the EFIS.
Anyway...... Reading the sensor output to the RDAC is where I'm going to start. It's either pulsing steady and changes with the flow of fuel or it's not. If I'm reading bias voltage and no or intermittent pulses it's a bad Red Cube.
I was PM's by a VAF reader that had to increase his bias voltage from 5 volts to 12 volts to get a more consistent reading from his Red Cube. If this is the case then it could be the RDAC is not getting a strong enough swing in voltage or the Red Cube is leaking light to the phototransistor or the transistor is not fully open and closed during a pulse. This is a next step after scoping the signal. One way or another I will get to the bottom of this.

Last edited by FasGlas : 12-26-2018 at 08:35 AM.
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  #37  
Old 12-26-2018, 02:27 PM
BobTurner BobTurner is offline
 
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Maybe that’s your problem (and solution). I found an (older) EI manual that specified the voltage to the red wire as 9 -15 volts, e.g., 5 volts may be too low for reliable operation.
Edit. I see someone else also suggested this.
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  #38  
Old 12-26-2018, 02:48 PM
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FWIW: The EI FT-60 "Red Cube" Installation Guide guide I have dated 3/11/13 states the input voltage must be between 8 - 30VDC.

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  #39  
Old 12-26-2018, 04:04 PM
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  #40  
Old 12-26-2018, 09:49 PM
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The image that Dan posted, presumably from MGL, shows a +12 volt supply to the red cube. Same as what Dynon, GRT and perhaps others supply from their engine monitors as a regulated output for driving fuel flow sensors. So that doesn't appear to be the issue here. It's the bias voltage being applied to the output that's in question. Dynon and GRT both have fuel flow inputs on their engine monitors that are simple one wire hookups (no external bias resistor) so the bias voltage they presumably apply to the signal input isn't obvious.

I too have had a red cube fail, though in my case I think I cooked it due to mounting location. Even so it went to almost 500 hours. Because I had a spare on the shelf I never bothered to determine if it was botched wiring as opposed to something else. Replacement is now up to about 250 hours. I've installed several on other RVs now with a combined several thousand hours of flight time and no failures, all with GRT or Dynon electronics to read the outputs.

Would be interesting to know what bias voltage the two above-mentioned manufacturers supply, and why. Not that it explains the failures...
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